1. Field of the Invention
This invention relates to liquid crystal compositions comprising compounds having a silacyclohexane ring, which a cyclohexyl group with a substituent is substituted on silane, and liquid crystal display elements containing the same.
2. Description of the Related Art
Liquid crystal display elements are elements utilizing the optical anisotropy and dielectric anisotropy possessed by liquid crystal materials. According to the display mode, there are various types including the TN (twisted nematic), STN (super-twisted nematic), SBE (super-twisted birefringent effect), DS (dynamic scattering), guest-host, DAP (distortion of aligned phase) and OMI (optical mode interference) types. The most common display devices are ones having a twisted nematic structure on the basis of the Schutt-Hellfritt effect.
Although the properties required for the liquid crystal materials used in these liquid crystal display elements more or less vary according to the display mode, a wide liquid crystal temperature range and stability to moisture, air, light, heat and electric fields are commonly required for all display modes. Moreover, it is desired that the liquid crystal materials have low viscosity and provide a short addressing time, a low threshold voltage and high contrast in the cells. At present, there is no single compound that meets all of these requirements, and it is a matter of fact that liquid crystal mixtures obtained by mixing several or several tens of liquid crystal compounds and/or latent liquid crystal compounds are being used. Accordingly, it is also important that these components are easily miscible with each other.
Among the various display modes, the active matrix (AM)-driven twisted nematic (TN) mode using a thin film transistor (TFT) element array or metal insulator metal (MIM) element array is being extensively employed owing to its high image display quality (i.e., high fineness, high contrast and high response speed). In this back ground, DRAM manufacturing technology in the industrial field of silicone semiconductor has been applied for liquid crystal manufacturing technology, resulting in acceleration of saving cost and technological progress.
In addition to the above-required properties, a nematic liquid crystal material used for the active matrix liquid crystal display (AM-LCD) is further required to have what is called a signal voltage holding property from reasons characteristic of this driving method. This signal voltage holding property designates the degree of drop of the signal voltage applied to TFT picture elements containing a liquid crystal material, within a given frame period. Accordingly, when the signal voltage does not drop (i.e., when the voltage holding ratio is 100%), the alignment of the liquid crystal molecules is not broken and, therefore, no reduction in contrast is caused. Moreover, this voltage holding property is affected by the environment in which the liquid crystal panel is used, so that the lifetime of the property tends to be shortened in an environment exposed to high-intensity light as in liquid crystal panels for projection use, and in an environment exposed to high temperatures as in liquid crystal panels for use on automobiles.
From these points of view we have had a patent application relating to the liquid crystal composition having a low driving voltage and high voltage holding rate for use of an AM drive because dielectric constant anisotropy has a positive value and a molecular structure of a component molecular contains a silacyclohexane ring (Japanese Patent Application Nos. 6-336184/1994 and 6-336185/1994).
It has been desired to lower the driving voltages of liquid crystal materials because the driving voltages of driver ICs have been lowered in the liquid crystal module. The liquid crystal compound having a silacyclohexane ring in a molecular structure is an extremely effective component for use by low-voltage driving as shown in aforementioned application. One of the important main components for the composition of aforementioned application is shown below: ##STR2## wherein R is an alkyl group of 1 to 7 carbon atoms, alkoxyalkyl group of 2 to 7 carbon atoms, a mono- or difluoroalkyl group of 2 to 7 carbon atoms, or alkenyl group of 2 to 7 carbon atoms,
n is 0 or 1, PA1 one of ##STR3## and ##STR4## is a trans-1-sila-1,4-cyclohexylene group or trans-4-sila-1,4-cyclohexylene group, while the other is trans-1,4-cyclohexylene group, PA1 X is H, F, Cl, OCHF.sub.2, OCF.sub.3, CF.sub.3, OC.sub.k H.sub.2k+1 or C.sub.k H.sub.2k+1 {wherein k is an integer in the range of 1 to 5}, (O).sub.s C.sub.p H.sub.q F.sub.r {wherein s is 0 or 1, p is 2, 3 or 4, q and r are each 0 or a positive integer, and (q+r)=(2p+1)}, (O).sub.s CY.dbd.CX.sub.1 X.sub.2 {wherein X.sub.1 and Y are each H, F or Cl, and X.sub.2 is F or Cl}, PA1 Y.sub.1 and Y.sub.2 are each H or F. PA1 n is 0 or 1; PA1 steric configurations of a silacyclohexylane ring and a cyclohexane ring are both in trans form; PA1 X is H, F, Cl, OCHF.sub.2, OCF.sub.3, CF.sub.3, OC.sub.k H.sub.2k+1 or C.sub.k H.sub.2k+1 {wherein k is an integer in the range of 1 to 5}, (O).sub.s C.sub.p H.sub.q F.sub.r {wherein s is 0 or 1, p is 2, 3 or 4, q and r are each 0 or a positive integer and (q+r)=(2p+1)}, or (O).sub.s CY.dbd.CX.sub.1 X.sub.2 {wherein s is 0 or 1, X.sub.1 and Y are each H, F or Cl, and X.sub.2 is F or Cl}; PA1 Y.sub.1 and Y.sub.2 are each H or F. PA1 m is 0 or 1; PA1 n is 0 or 1; ##STR9## are each a trans-1-sila-1,4-cyclohexylene, trans-4-sila-1,4-cyclohexylene or trans-1,4-cyclohexylene group; PA1 X is H, F, Cl, OCHF.sub.2, OCF.sub.3, CF.sub.3, OC.sub.k H.sub.2k+1 or C.sub.k H.sub.2k+1 {wherein k is an integer in the range of 1 to 5}, (O).sub.s C.sub.p H.sub.q F.sub.r {wherein s is 0 or 1, p is 2, 3 or 4, q and r are each 0 or a positive integer, and (q+r)=(2p+1)}, or (O).sub.s CY.dbd.CX.sub.1 X.sub.2 {wherein s is 0 or 1, X.sub.1 and Y are each H, F or Cl, and X.sub.2 is F or Cl}; PA1 Y.sub.1, Y.sub.2, Y.sub.3, Y.sub.4, are each H or F. PA1 (a) Alkyl groups of 1 to 7 carbon atoms, including methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl and n-heptyl. PA1 (b) Alkoxyalkyl groups of 2 to 7 carbon atoms, including methoxymethyl, 2-methoxyethyl, 3-methoxypropyl, 4-methoxybutyl, 5-methoxypentyl, 6-methoxyhexyl, ethoxymethyl, 2-ethoxyethyl, 3-ethoxypropyl, 4-ethoxybutyl, 5-ethoxypentyl, (n-propoxy)methyl, 2-(n-propoxy)ethyl, 3-(n-propoxy)propyl, 4-(n-propoxy)butyl, (n-butoxy)methyl, 2-(n-butoxy)ethyl, 3-(n-butoxy)propyl, (n-pentoxy)methyl, 2-(n-pentoxy)ethyl and (n-hexyloxy)methyl. PA1 (c) Mono- and difluoroalkyl groups of 2 to 7 carbon atoms, including 1-fluoroethyl, 1-fluoropropyl, 1-fluorobutyl, 1-fluoropentyl, 1-fluorohexyl, 1-fluoroheptyl, 2-fluoroethyl, 2-fluoropropyl, 2-fluorobutyl, 2-fluoropentyl, 2-fluorohexyl, 2-fluoroheptyl, 3-fluoropropyl, 3-fluorobutyl, 3-fluoropentyl, 3-fluorohexyl, 3-fluoroheptyl, 4-fluorobutyl, 4-fluoropentyl, 4-fluorohexyl, 4-fluoroheptyl, 5-fluoropentyl, 5-fluorohexyl, 5-fluoroheptyl, 6-fluorohexyl, 6-fluoroheptyl, 7-fluoroheptyl, 1,1-difluoroethyl, 1,1-difluoropropyl, 1,1-difluorobutyl, 1,1-difluoropentyl, 2,2-difluoroethyl, 2,2-difluoropropyl, 2,2-difluorobutyl, 2,2-difluoropentyl, 3,3-difluoropropyl, 3,3-difluorobutyl, 3,3-difluoropentyl, 4,4-difluorobutyl, 4,4-difluoropentyl and 5,5-difluoropentyl. PA1 (d) Alkenyl groups of 2 to 7 carbon atoms, including vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4E-hexenyl, 4Z-hexenyl, 4E-heptenyl, 4Z-heptenyl, 5-hexenyl, 5E-heptenyl, 5Z-heptenyl and 6-heptenyl. PA1 (a') Alkyl groups of 2 to 7 carbon atoms, including ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl and n-heptyl. PA1 (b') Alkoxyalkyl groups of 2 to 7 carbon atoms, including methoxymethyl, 2-methoxyethyl, 3-methoxypropyl, 5-methoxypentyl, ethoxymethyl, 2-ethoxyethyl, (n-propoxy)methyl and (n-pentoxy)methyl. PA1 (c') Mono- and difluoroalkyl groups of 2 to 7 carbon atoms, including 2-fluoroethyl, 2-fluoropropyl, 4-fluorobutyl, 4-fluoropentyl, 5-fluoropentyl, 6-fluorohexyl, 7-fluoroheptyl, 1,1-difluoroethyl, 2,2-difluoroethyl, 2,2-difluoropropyl, 2,2-difluorobutyl, 4,4-difluorobutyl and 4,4-difluoropentyl. PA1 (d') Alkenyl groups of 2 to 7 carbon atoms, including vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4E-hexenyl, 4Z-hexenyl, 4E-heptenyl, 4Z-heptenyl, 5-hexenyl and 6-heptenyl. PA1 T.sub.NI : Nematic-isotropic transition temperature (.degree. C.). PA1 V.sub.th : Threshold voltage PA1 T.sub.on : initial response time (msec) PA1 T.sub.off : trailing response time (msec) PA1 VHR: Voltage holding ratio.
As one of the examples for the above ring structure in the aforementioned application, using the compound ##STR5## lowering voltage has been achieved.
A liquid crystal compound containing a silacyclohexane compound is a useful component for lowering voltage in comparison with the conventional hydrocarbon liquid crystal compounds. However, it has disadvantage of being slower in response speed and lower in T.sub.NI (nematic-isotropic transition temperature) than the conventional liquid crystal compounds.